Introduction

Brainstem and cerebellar lesions are commonly associated with disorders of extraocular movements and these have localising value. The dorsal midbrain syndrome is characterised by (1) impaired voluntary vertical eye movements, (2) light-near dissociation of the pupillary response (pseudo-Argyll Robertson pupils), (3) convergence nystagmus on attempted upward gaze, (4) lid retraction (Collier sign) and (5) skew deviation.1 ,2 This syndrome is usually produced by pressure on the midbrain tectum.2 In this syndrome, the mesencephalic reticular formation that includes the rostral interstitial nucleus of medial longitudinal (riMLF) fasciculus and its connection with the interstitial N of Cajal and Darkschewitsch and the posterior commisure are involved. The riMLF is located dorsomedial to the rostral end of the red nucleus, rostral to the oculomotor nucleus and ventral to the periaqueductal grey matter (figure 1).3